In general, an anode-supported solid oxide fuel cell (SOFC) has a ceramic multilayer structure including an electrolyte having a thickness of several tens of micrometers, a reaction-preventing layer and a cathode, stacked successively on an anode support having a thickness of about 1 mm.
To form a dense and firm electrolyte thin film layer in such a ceramic multilayer structure, it is essentially required to control the sintering characteristics of raw material powder and optimize them. If not, a difference in sintering shrinkage behaviors between an anode and an electrolyte, results in bending of a cell upon simultaneous sintering, which may cause various types of processing defects or failure of a cell. In addition, when a thin film layer is formed by post-sintering, it is not possible to increase the density of a film sufficiently at a low degree of sintering. On the contrary, when a degree of sintering is excessively high, a film excessively shrinks before an interfacial binding with a substrate is formed, resulting in interfacial defects such as delamination. Therefore, when an electrolyte thin film is manufactured through post-sintering, it is very important to control the sintering characteristics of an electrolyte in order to densify the thin film layer, while inhibiting generation of processing defects.
Commercially available electrolyte powder includes yttria-stabilized zirconia, scandia-stabilized zirconia, gadolinia-doped ceria and samaria-doped ceria. Such powder has its unique defined specification. Thus, it is very difficult to control the sintering characteristics to meet a particular use by using such powder.
Therefore, conventional methods for controlling a degree of sintering of a sintered body, such as ceramic, include controlling a powder size or adding a sintering aid. It is possible to increase a degree of sintering using nanoparticles having a large specific surface area. However, it is very difficult to synthesize particles while controlling the particle size gradually from a nanometer scale to a micrometer scale. Thus, it is practically impossible to obtain sintering characteristics required for a particular situation precisely by controlling the particle size itself. In addition, it is possible to use a method of mixing crude particles with fine particles to control a degree of sintering gradually. However, fine nano-scaled powder has strong cohesive force, and thus it is difficult for such nano-scaled powder to be dispersed completely among crude particles. When such nanopowder forms agglomerate, it cannot provide a function of improving a degree of sintering. Moreover, a portion where nanopowder is agglomerated shows a relatively lower sintering rate as compared to the other portions. Therefore, processing defects caused by a local difference in sintering characteristics may occur with ease.
Another method for improving the sintering characteristics of the sintered body is adding a sintering aid. However, such a method has a problem in that it is not possible to form a dense film uniformly due to a local difference in sintering rate. In general, the sintering aid is added in the form of an oxide or nitrate. When the sintering aid is not distributed uniformly, a portion where a large amount of sintering aid is present shows a high densification rate, while the other portion causes a processing defect, such as pores in the vertical direction.
Under these circumstances, it is required to provide a method for distributing nanopowder and a sintering aid very uniformly and to inhibit agglomeration in order to obtain a film having a desired structure by controlling sintering behaviors and inhibiting generation of processing defects by using the effects of the nanopowder and sintering aid.